Nuclear magnetic resonance ("NMR") phenomena cause the human body (or other object of interest) to generate radio signals ("NMR signals") for pick-up by a magnetic resonance imaging ("MRI") system. The magnetic resonance imaging ("MRI") system has a radio frequency (RF) receiver which receives, amplifies and filters these NMR signals. The RF receiver's output is processed by a computer to generate a displayed image of the body's internal structure.
To generate high quality images, it is important for the MRI RF receiver to accurately receive and process the weak NMR signals generated by the body. Unfortunately, various defects in RF receiver operation (e.g., internally generated noise, digital quantization errors, "aliasing" effects, quadrature channel imbalance) can introduce distortion and noise which degrade the "purity" of the received NMR signals--and thus decrease the quality of the resulting images.
Until very recently, analog receivers were used to receive the NMR signals in MRI imaging systems. One weakness in past analog receiver design relates to the anti-aliasing filters that must be used prior to digital sampling to prevent out-of-band signal components from being "folded over" into the desired signal band during the sampling process. These filters must provide tight frequency selectivity and other high performance, and therefore are expensive to design and implement while nevertheless suffering from certain performance deficiencies (e.g., frequency-dependent phase shifts, poor transient response, non-zero DC offset, drift, and imperfect gain and phase matching between pairs of filters used for "quadrature" channels).
So-called "digital receivers" have replaced analog receivers in other applications (e.g., radar and communications) requiring high-performance signal processing. As digital signal processors (DSPs) and other "fast" digital hardware have become available, cost-effective and increasingly more capable, it has become possible to filter the received NMR signal in the digital domain instead of the analog domain in order to increase receiver performance in certain respects. For example, it is possible to construct, using modern digital signal processing components, a digital filter which exhibits a linear phase response throughout its passband. Digital filtering provides additional significant advantages including, for example, programmability, potential increases in dynamic range, and potential improvement in S/N. Similarly, "digital receivers" implemented using modern digital signal processing components have several well known advantages such as elimination of DC and low frequency noise, perfect matching of I and Q (quadrature) channels, high performance filters and finer quantization. Digital signal processing techniques can thus significantly improve noise and distortion performance (and hence image quality), and can provide additional advantages such as increased stability and flexibility.
Despite the apparent applicability of digital receiving techniques to MRI systems, MRI imposes certain performance constraints that are not commonly found in other types of RF systems that use digital receiving techniques. For example, a typical digital RF communications receiver continuously processes a received input signal for a relatively long time (e.g., seconds, minutes, hours, or even longer). Commonly, the digital receiver "locks" onto the phase of the input signal to ensure that the receiver is continually synchronized with the phase of the signal. For example, a digital "phase locked loop" detector may be used to extract phase information from the incoming input signal. This phase lock is then used to synchronize the detector with the phase of the incoming signal and thereby ensure that the detector does not introduce phase errors into its output. Because the input signal is continuously available, the digital receiver's phase locked loop can unambiguously "follow" the phase of the input signal so long as the signal continues to be present.
In contrast, in a MRI system the input signal is only intermittently available to the RF receiver due to the excitation/acquisition cycling inherent in the NMR process. At least part of the RF receiver must be turned off during each RF pulse spin excitation in an MRI system in order to prevent the receiver from becoming desensitized (or even damaged) by the powerful RF transmit pulse. The receiver is turned on between RF pulses to receive the NMR signal "echo" emitted by the nuclei as they return to spin-lattice equilibrium. The NMR signal of interest consists of one or more energy "bursts" combined with noise. Since phase encoding techniques are often used to provide multidimensional imaging, many different NMR signals of different phases may be received at more or less the same time during a given acquisition cycle. The MRI receiver must accurately receive each entire NMR signal envelope without introducing any phase ambiguities into the received signal--even though sometimes there is only a relatively short time (e.g., much less than a millisecond) between when the RF transmitter has finished transmitting and the time when the receiver must receive the responsive envelope of NMR signals.
To make matters even more difficult, an MRI "experiment" used to collect dam for an image may consist of hundreds or thousands of such NMR excitation/acquisition cycles spanning a duration of tens of minutes. In view of the importance of NMR signal timing characteristics to the imaging process, it is critical that the receiver output remains "phase locked" for the entire duration of a complete experiment--a difficult objective to achieve considering that the signal to be received is only intermittently available, and that the receiver "front end" must be deactivated each time the RF transmitter is transmitting. The MRI receiver cannot change its phase shift from one acquisition cycle to another, since any such inconsistent phase shift would destroy the phase coherence of the acquired data set and degrade the spatial resolution and/or clarity of the resulting image or could introduce annoying artifacts into the image.
In addition, all time delays introduced by digital processes (e.g., digital receiver detection and filtering) must be completely predictable and compensated for so that there is no loss of signal timing information. Uncompensated delays introduced by the digital filtering process (or at any stage along the signal processing path) have the potential for introducing artifacts or decreasing the spatial resolution of the final image. Thus, the receiver must be carefully designed so that timing parameters associated with all of the data in the data set produced by an experiment are internally self-consistent, and fully synchronous with MRI sequencer control signals.
Because of the often rapid, repeated cycling between the "excitation" and the "acquisition" parts of the MRI imaging process, it is necessary for the digital MRI receiver to finish processing a previously received NMR signal before the next NMR signal arrives. All digital filtering and detection should be completed within about 1 msec (the minimum time between successive signal receptions in a modern MRI system) so that the receiver is ready to receive the next NMR signal envelope. In view of the computational complexity of the necessary digital filtering, it is difficult to provide a system that can complete all digital filtering and detection in this short time period. Typical digital signal processors (DSPs) are capable of performing only a single FIR (finite impulse response) filtering calculation per clock cycle. The number of filtering calculations involved in processing a signal through an FIR digital filter is related to the number of filter "taps" (analogizing to old-style "tapped" delay lines). Generally, more "taps" are required to provide higher performance with respect to certain filter parameters (e.g., bandwidth and/or sharpness of roll-off). Cascading two or more FIR digital filters in a "multi-rate" arrangement may decrease the overall number of computations and thus reduce the total number of "taps" through the FIR digital filtering process. However, even cascaded digital filters typically require several hundred "taps" to achieve the pass band and roll off characteristics necessary to adequately filter an NMR signal prior to digital re-sampling (decimation) or sampling rate reduction.
Some work has been done in the past to solve problems associated with using a digital receiver in an MRI system. As one example, it is known to operate the quadrature detector (demodulator) of an MRI digital receiver from the same constant stable time base used to generate the MRI transmitter carrier frequency in order to insure phase consistency of the received NMR signals in multiple imaging sequences. However, further improvements are possible.
In accordance with one aspect of the present invention, a multi-rate FIR filter MRI digital receiver accurately maintains synchronization between the ongoing NMR phenomena and signals digitally processed in real time--despite the intermittent availability of the NMR signal and despite intermittent operation of at least parts of the digital receiver used to receive and process the NMR signal. In accordance with this aspect of the present invention, a phase locked relationship is established between the signal received and processed by the digital receiver, and the physical NMR process the body being imaged is undergoing. Once established, the phase/timing relationship is maintained for the duration of the particular NMR experiment being performed. In addition, special logic in the digital signal processing system ensures that the digital receiver's data output is fully synchronized with an external synchronous signal controlling data acquisition within the MRI system. This is important in allowing images free of artifacts to be computed.
In accordance with the phase locked aspect of the present invention, the MRI digital receiver establishes the phase of the received NMR signal and processes it in a phase locked, time-coherent fashion. This phase locking in the preferred embodiment basically involves: (a) establishing a fixed, ascertainable timing relationship between periodic digital receiver input signal sampling times and the phase of a periodic transmitter carrier signal; and (b) digitally processing (e.g., filtering) the resulting sampled values in a manner that observes this fixed phase relationship in order to ensure that the digital process output is phase-consistent across all NMR signal acquisition cycles throughout an MRI experiment.
In order to observe the fixed phase relationship across all NMR signal acquisition cycles within an MRI experiment, the present invention provides a phase-maintaining mechanism (e.g., a sequential state machine such as a counter or other logic device capable of keeping track of periodic phase state changes) that is active throughout the MRI experiment. This phase-maintaining mechanism continuously keeps track of the RF carrier phase even during RF transmit times when the digital receiver is nominally "off". The phase-maintaining mechanism is used to synchronize further digital processing. For example, the phase-maintaining mechanism can impose phase synchronization on the quadrature detection process (and thus also on the subsequent digital filtering process) to ensure that these processes are performed in a phase-consistent manner from one acquisition cycle to the next throughout the entire MRI experiment.
In the preferred embodiment, analog-to-digital conversion sampling is performed continuously--even during RF pulse transmit times when the digital receiver is nominally "off". The analog-to-digital converter sampling timing is synchronized with a continuous reference time base used to generate the transmitter carrier frequency. This has the effect of establishing a predetermined fixed relationship between (a) the RF carrier phase/frequency, and (b) the digital sampling rate of the analog-to-digital converter. Because the sampling process is continuously synchronized with the phase of RF transmitter pulses used to generate NMR phenomena within the body being imaged, the sampling process is inherently synchronized also with those NMR phenomena--and with the resulting NMR signals emanated by the body and received by the digital receiver. This means that each of the digital data values output by the analog-to-digital converter results from sampling at a specific predetermined fixed--and ascertainable--timing relative to the phase of the NMR phenomena occurring within the body being imaged. Once "phase lock" has been established, a "receive window" is opened during each NMR signal acquisition cycle when the output of the digital receiver is valid.
To establish "phase lock", the preferred embodiment assigns the first output of the analog-to-digital converter as the phase "zero" reference, and then continues to digitize data during an entire MRI protocol ("experiment"). This means that the sampling process provides a stream of data points corresponding to a periodic, unbroken sequence of time instants during an entire MRI experiment--with those time instants being synchronized with particular phases of the RF carrier frequency. The digital receiver identifies the phase of the received signal at an arbitrary cycle as the "zero" phase reference. The phase points corresponding to the periodic samples repeat for each periodic cycle of the transmitter carrier frequency. Therefore, the preferred embodiment provides what in one implementation is essentially a counting function for correlating or indexing each successive sample relative to the arbitrary "zero" phase reference point of each RF carrier period. A "phase index" can then index each successive digital sample point relative to the arbitrary "zero" phase reference.
In the preferred embodiment, an integer ratio (e.g., 4 to 1) exists between the sampling frequency and the down-converted RF carrier frequency as it is provided at the output of the last intermediate frequency (IF) section of a super-heterodyne conversion process. This means that there is a fixed ratio of four samples for each IF carrier cycle--and moreover (since the phase of the sampling process is locked to the phase of the carrier frequency) that the sample timing corresponds exactly to four different specific phase angles (e.g., 0.degree., 90.degree., 180.degree. & 270.degree.) defined relative to the period of the IF carrier frequency. The absolute sample times need not correspond to the absolute phase of the carrier signal--what is important is that the timing relationship between the sample times and carrier phase is fixed and predetermined for the duration of the MRI experiment.
The present invention provides several alternative techniques/structures for implementing a mechanism for keeping track of the carrier signal phase and maintaining relative synchronization between that phase and the digital processing in order to maintain "phase lock" throughout an MRI experiment. All of the alternative techniques allow a "phase index" to specify either explicitly or implicitly the phase relationship between the phase reference point and each successive digital sample in the input data stream.
In accordance with one alternative, the digital receiver quadrature demodulator is operated continuously during an entire MRI experiment. By continuing to process, even during times when the data is non-valid (e.g., during the excitation stage when the RF transmitter is activated), the digital receiver demodulator never loses synchronization and therefore does not have to be resynchronized upon reception of each new NMR signal. In accordance with one aspect of the preferred embodiment of the present invention, a separate digital signal processor is provided with a dedicated function of maintaining phase lock and demodulating the NMR signal. This DSP is always active during operation of the digital receiver, and therefore it never misses an input and thus maintains continuous synchronization. One or more further DSPs (which can be active only during times when the transmitter is not transmitting) may be provided to do the digital filtering analysis.
Another alternative technique for maintaining synchronization throughout an entire experiment is to provide a dedicated counter which continually counts the number of digital samples. If the analog-to-digital conversion frequency is locked to the RF carrier frequency, the counter can increment for each predetermined angular displacement of the received IF carrier signal (e.g., every 90.degree.). The counter output may then be used to keep track of the signal phase. Since the analog-to-digital converter and associated phase-counting counter operate all the time throughout an MRI experiment, phase coherency is never lost. The rest of the digital receiver (e.g., the demodulator and the digital filter processes) can be turned on and off as needed throughout the experiment.
Another alternative technique is to synchronize the "receiver on" timing control signal with the phase of the analog-to-digital conversion clock. Using this technique, the MRI microcoded digital sequencer used to generate the "receiver on" timing control can be synchronized with the phase of the analog-to-digital conversion clock so that it always turns the receiver "on" at a consistent predetermined phase relative to the RF carrier. This ensures that the digital receiver will always process the digital samples in a phase-coherent manner from one acquisition cycle to the next. Such synchronization establishes relative phase lock within the measurement process by ensuring that the digital receiver always becomes active at a predetermined phase relative to the sampled outputs generated by the analog-to-digital converter. If this technique is employed, care must be taken to ensure that the "receiver on" control signal does not interfere with the dam input due to digital logic "race conditions."
Another aspect of the present invention is the data acquisition synchronization capability which is implemented in the multi-rate FIR digital filter. Synchronization is important for MRI due to the collection of a large number of data sets which must be precisely time related. Data is collected over the duration of the experiment but data sets will all be processed together using a 2D or 3D FFT after the MRI sequence is completed. With an analog filter, the group delay is well defined so that a signal appears at the output with a fixed delay after the input has occurred and the data timing with respect to the leading edge of the "SAMPLE GATE" signal (during which received data is valid) is maintained for every data acquisition time window throughout the MRI sequence. With digital filters, the group delay is also fixed but it is necessary to determine the exact data input coinciding with the SAMPLE GATE leading edge and to track it throughout the multi-stage FIR filtering process. Time synchronization of an FIR input to an external control signal and correlation to a specific output is not provided for in the standard convolution of a signal with an FIR filter, which normally assumes a continuous input signal. The present invention provides that the first output of the FIR multi-rate filter is precisely related in time to the specific input data point which occurred at the leading edge of the SAMPLE GATE signal generated by the MRI sequencer controller. The input data index in the buffer corresponding to the leading edge of SAMPLE GATE is saved as the "sync" index for the x[ ] (first FIR filter stage input) data buffer. Decimation of the first FIR stage data output y[ ] requires the calculation of the "sync1" index for the yd[ ] buffer of decimated data used as input to the second FIR filter stage. In the case of a two stage multi-rate FIR filter, the first decimated output of the second FIR filter stage zd[ ] is exactly time synchronous with the original x[sync] input data value. The efficient implementation of data acquisition synchronization in the multi-rate FIR filters is important and ensures MRI images free of artifacts.
In accordance with another aspect of the present invention, digital filtering is optimized to minimize the time ("latency") between the end of the SAMPLE GATE acquisition control signal and the time at which the last output from the digital signal processing system is ready. This technique provided by the present invention minimizes how much time a multi-stage FIR digital filtering system requires to process an NMR signal envelope. The standard approach for computing cascaded FIR filters is to perform the filtering convolution computations serially. After enough data points from the input stream are collected, the first FIR filter stage outputs are computed. Once enough outputs of the first filter stage are computed to allow a standard convolution, then successive outputs of the second filter stage are computed. In either case for a real time system, this means that there is a significant lag in the generation of filter outputs after the final data has been input to the multi-stage filter since each output requires a full standard convolution to be performed. In fact, it is not necessary to wait until all data points have been collected before FIR filter convolution computations can be performed. Once the " synchronization index" into the input data stream has been identified by the leading edge of SAMPLE GATE, it is possible to predict the set of indices which will contribute to each output of a FIR filter.
An "inverted convolution" is used in the preferred embodiment to determine which products involving a particular filter input are part of the sums forming any particular filter output. This makes it possible to compute the products of the summation for each filter stage output as the data becomes available to perform each computation. In other words, it becomes possible to compute an array of partial sums, one for each output, on an ongoing basis (with each newly available input being used to compute appropriate products that are added to the array of partial sums on a "rolling basis") such that some subset of the partial sums are approaching a final value over the duration of the data acquisition. Once the last input becomes available, the digital filter only has to calculate n products and n additional partial summations (where n is less than or equal to the number of filter "taps")--instead of n standard convolutions as would normally be required--before the final subset of filter outputs are computed completely. Note that in the preferred embodiment it is assumed that after the last data input and the de-assertion of SAMPLE GATE, all subsequent input data x[ ] are zero. There is thus minimal time lag between when the last valid input is applied to the digital receiver and when the digital receiver can finish computing and outputting its last valid digitally filtered data.
By minimizing the latency between the time the last filter input becomes available and the time the digital filter is able to output final, digitally filtered results, the digital receiver provided by the present invention finishes processing the NMR signal envelope quickly even though the digital receiver is applying very complex digital filtering to the incoming signal. Minimum time lag is important because the time between successive RF excitation/acquisition cycles for certain types of MRI sequences can be very short (e.g., on the order of 1 ms)--and the digital receiver must finish processing a previous NMR signal envelope before a next successive signal envelope arrives. The minimal digital filtering time provided by the present invention allows relatively complicated digital filtering using "long" or large order (i.e., large number of "taps") FIR digital filters to be used for excellent digital filtering characteristics. Out-of-band noise is substantially eliminated to improve image clarity while image flatness and field of view are maximized. Even though large order digital filters are used, the MRI system is ready to begin processing the next successive NMR signal envelope at least by the time the next NMR signal envelope begins to arrive.